it sounds like a v-brake noodle. Course you have cantis -it might just be there by mistake (pure speculation on my part, but that's what boards are about, right?)

I have seen plastic like sheathing run under the bb before, but from your description it doesn't sound right. Also on one of my bikes (older one) there is a noodle like bit that has the brake cable that runs through a metal housing just before the rear brake, but from pictures of the Windsor Tourist, it doesn't look like that's the case either.

OK, bets are open: I'm going with v-brake noodle in there by mistake. Anyone else?